This invention relates to the technology of free-electron lasers featuring wavelength tunability, high output power, high efficiency, ultra-short pulses and high quality. These features of free-electron lasers can be effectively used in various industrial fields including medical diagnosis (of cancer, specific metabolites, erythrocyte sedimentation rate, optical CT imaging), therapeutic means (spalling of gallstone, dental caries, laser knife, laser drill, disinfection), cutting/fusing/welding (of ceramics, metal working, ship building, construction, chemical plants), modifications (of polymers, fibers, dyeing), ultra-high sensitive detection (of environmentally-harmful materials, global warming substances), decomposition (of toxic gases, environmentally-harmful materials, global warming substances), separation (elements, isotopes, refining of pharmaceuticals, removal of harmful substances), synthesis (of pharmaceuticals and thin films), energy beaming (unmanned high-altitude meteorological observers platform, communication satellites, interplanet space vehicles and probes, laser thrusting for rockets, space debris, satellites, comets, airplanes, ships and other vehicles), energy production (laser nuclear fusion, muon-catalyzed nuclear fusion), lighting sources (for detection and large-scale illumination) and heating sources (for disinfection, sterilization, decommissioning or nuclear reactors, decontamination of nuclear reactors, heat treatment, annealing, precipitation, large-scale heating). The invention relates particularly to a method and an apparatus for realizing high-extraction efficiency of laser light from electron beams in free-electron lasers with a view to achieving significant improvements in such factors as laser construction and operating costs, specific capacity per output and overall efficiency.
In free-electron lasers, the extraction efficiency of laser light from electron beams is expressed by 1/(2 Nw)˜1/(4 Nw) (Nw=the number of undulator periods). In order to increase this extraction efficiency, conventional free-electron lasers have used very short undulators (with a smaller number of periods) at the sacrifice of the laser gain.
Similarly, tapered undulators have been used at the sacrifice of laser gain, with the magnetic field or the length of undulator periods being varied either through a plurality of stages or in a continuous manner. As the electron energy decreases due to the transfer of energy from electron beams to laser light, the free-electron laser is decoupled from the conditions for resonance and electron beam energy are no longer transformed to laser light energy. In order to avoid this problem, many attempts have been made to increase the limit of the extraction efficiency of the laser light from the electron beams by changing the magnetic field on undulators or the length of undulator period.
In these methods, the inherently small gain of the free-electron laser is significantly decreased, the required experimental conditions cannot be stably realized in a consistent manner and the energy tunability is sacrificed. Because of these practical disadvantages and difficulties, the methods have not been applicable except for special and impractical experiments. In fact, oscillation of high-intensity laser light producing high average power using these methods has heretofore been limited to the conceptual and no practical equipment has been realized. Therefore, in the prior technologies, in order to facilitate reliably and easily the oscillation of laser light in the free-electron laser, its gain has been increased at the sacrifice of the extraction efficiency of laser light from electron beams. In other words, there had been no specific method by which the extraction efficiency can be increased beyond its limit while maintaining high laser gain.
Another problem with the conventional free-electron laser is that it emits optical pulses slightly longer or shorter than electron pulses with width of a few to several hundred picoseconds. Hence, there has been no specific way to fabricate a high-power, high-efficiency laser capable of generating ultra-short pulses in the femto-second range including the free-electron laser.
Speaking of solid lasers, they produce low average power with low efficiency at fixed wavelength and the only practical proposal that has been made so far is for Ti:sapphire lasers to produce a large peak power of the tera-watt class in single shots of pulse shorter than 100 femto-seconds and a continuous wave of extremely low peak power.